Story: Soil erosion and conservation

Page 7 – Biological control of erosion

A range of measures have been used to control or prevent
erosion on New Zealand’s farmland. Some were adopted from
other countries such as the USA, some were learnt by trial
and error, and others developed from research.

Biological control is usually cheaper, but more risky,
than mechanical methods.

Trees

Trees reduce the rate of erosion by:

protecting the soil from the impact of rain

transpiring large amounts of water, which counteracts
very wet soil

binding soil to sloping land with their roots.

Spaced planting

Trees, commonly poplars, are planted strategically on
slip-prone hill country such as the area around Taihape.
Thousands have also been planted throughout the North Island
hill country.

They are planted as poles, and protected from grazing
animals until established. Poplars are fast growing and their
root systems develop rapidly. But they are deciduous and so
do not transpire as much water in winter as evergreen
species.

Where trees can be easily harvested, pines (Pinusradiata) may be planted at a site where both pasture
and trees for timber are managed together. Known as
agroforestry, this can be successful on more productive sites
and less steep terrain, where trees generally grow
faster.

Pair planting

This method is used to control gully erosion, for both
permanent and ephemeral streams. Willows or poplars are
planted about 2–3 metres apart on each side of the gully
floor. In time their roots create a damming effect which
slows the water flow, stabilises the gully sides, and allows
other plants to regrow.

Stream bank planting

Willows are planted at the edge of streams and rivers to
stem the erosion of the banks. Traps or small dams may also
be used to catch material dislodged from the edge.

Riparian strip

This is a strip (10 or more metres wide) near a stream or
river, which is fenced to exclude farm animals. It may be
planted with a mix of trees, shrubs and pasture. The aim is
to trap sediment, and nitrogen and phosphorus in water
run-off, from nearby slopes before it enters waterways.

Plantation trees

Pines and cedars planted for forestry production (around
400 stems per hectare) can control slips and gully erosion on
hill country, if they cover an area larger than the gully.
Some 320 square kilometres were reforested through the East
Coast forestry project, begun in 1992.

Shelter belts

Rows of trees, planted at right angles to the prevailing
wind, help reduce wind erosion and protect livestock and
crops. Radiata pine (Pinus radiata) and macrocarpa
(Cupressus macrocarpa) are the most widely used
species.

Shelter belts can have more than one row, and more than
one species, and may be up to 10 metres wide. For best effect
they should be as long as possible – at least 12 times as
long as the height of the trees.

Protection forestry

Protection forestry is managed on land that has little
production value, but has considerable value as erosion
protection if fenced off from grazing animals. Often these
areas are the headwaters of large watersheds. They may
consist of regenerating forest or be planted in trees (exotic
or native).

Saved by trees

Research in the Gisborne region by Landcare scientist
Mike Marden showed that reforestation of gullies
successfully controlled erosion in all but the largest of
them. Unplanted gullies continue to erode.

Pasture

A dense and vigorous pasture will reduce sheet and wind
erosion, but its shallow roots do not prevent slips, gullies
and slumping. In the South Island high country, runholders
are encouraged to retire the upper catchment areas from
grazing and allow the snow tussock ecosystems to
regenerate.

Animal pest control

Controlling animal pests is important in reducing erosion
caused by farming. An example is Molesworth Station in
Marlborough, once a sheep station ravaged by rabbits. Sheep
were replaced by beef cattle, which do not graze as closely,
so they expose the pasture to rabbit invasion. Controlling
the rabbits helped in the revegetation of depleted hill
country.